chore(frontend): real ESLint instead of a placeholder; fix hook order (FLAW-19)

eslint.config.js with the recommended JS/TypeScript rules and the React
hooks rules; npm run lint runs ESLint and is part of the image build.
Errors fail the build, the 24 exhaustive-deps findings stay warnings.

It found two real bugs: TaskDetailModal and ManageColumnsModal called
their hooks after 'if (!isOpen) return null', so the hook count changed
whenever the dialog opened or closed. Both are now a thin wrapper plus a
content component. Also removes unused imports and dead code (old task
list share handlers, an unused realtime status, an unused rollup).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/triage/ManageColumnsModal.tsx

@@ -8,7 +8,6 @@ import {
   RotateCcw,
   Check,
   Columns,
-  Layers,
 } from 'lucide-react';
 import { KanbanColumn } from '../../lib/types';
 import { triggerHaptic } from '../../lib/api';
@@ -28,15 +27,17 @@ export const DEFAULT_KANBAN_COLUMNS: KanbanColumn[] = [
   { id: 'done', name: 'Erledigt', isDone: true, color: 'emerald' },
 ];
 
-export const ManageColumnsModal: React.FC<ManageColumnsModalProps> = ({
+// Wrapper and content split so the hooks always run in the same number (see TaskDetailModal)
+export const ManageColumnsModal: React.FC<ManageColumnsModalProps> = (props) =>
+  props.isOpen ? <ManageColumnsModalContent {...props} /> : null;
+
+const ManageColumnsModalContent: React.FC<ManageColumnsModalProps> = ({
   isOpen,
   onClose,
   initialColumns,
   onSave,
   listTitle,
 }) => {
-  if (!isOpen) return null;
-
   const [columns, setColumns] = useState<KanbanColumn[]>(() => {
     return initialColumns && initialColumns.length > 0 ? initialColumns : DEFAULT_KANBAN_COLUMNS;
   });